The present invention relates to shoe press units used, for example, in papermaking for pressing a paper web. The invention relates more particularly to a method and a shoe press unit in which excess lubricating oil that is expelled from between a pressing surface of a press shoe and a flexible belt is captured and evacuated from the shoe press unit.
A shoe press unit typically comprises a support beam, a shoe element movably supported on the beam, a pressing unit arranged between the beam and the shoe element for urging the shoe element away from the beam and toward a counter element such as a counter roll, and a flexible belt that is arranged to slide over the pressing surface of the shoe element. To reduce friction between the belt and the shoe element and thereby reduce the frictional heating of the belt, it is common to supply a lubricating oil between the pressing surface of the shoe element and the belt. The oil both lubricates and cools the belt and the pressing surface. Excess oil is expelled from between the belt and the pressing surface as a result of the pressure exerted in the nip between the shoe element and the counter element. The excess oil is expelled from an upstream edge region of the pressing surface, and is then evacuated from the shoe press unit by an oil evacuation arrangement.
U.S. Pat. No. 5,084,137 discloses a shoe press unit having an oil evacuation arrangement in which an inlet opening of the oil evacuation arrangement is formed in a lower integral part of the shoe element. The inlet opening is so located that the initial kinetic energy of the excess oil exiting from between the belt and pressing surface is lost before the excess oil passes through the inlet opening. Thus, this kinetic energy of the oil is not utilized to assist in evacuating the oil. Another disadvantage of the oil evacuation arrangement is that it does not prevent the excess oil from flowing in various directions within the shoe press unit, and hence the oil tends to accumulate in the shoe press unit. The accumulated oil tends to mix with air, which makes evacuation of the oil more difficult and also requires a subsequent processing of the evacuated oil to separate the air from the oil prior to reusing the oil. The accumulated oil, which is relatively hot because of the heat transfer from the belt to the oil, also tends to conduct heat to other parts of the shoe press unit before it is evacuated, which results in an undesirable temperature increase inside the shoe press unit. Moreover, it is disadvantageous to have an accumulation of oil in the shoe press unit because this requires an increased power consumption. Finally, constructing the oil evacuation arrangement as an integral part of the shoe element requires relatively costly manufacturing methods.
The present invention addresses the above and other needs by providing a method and a shoe press unit in which an oil evacuation arrangement is formed separately from the shoe element and is affixed to the shoe element proximate an upstream edge region of its pressing surface, such that the shoe element and oil evacuation arrangement move together as a unit. The oil evacuation arrangement has an inlet opening located with respect to the shoe element such that a major portion of the excess oil expelled from between the belt and the shoe element passes through the inlet opening with a kinetic energy that is substantially undiminished from the initial kinetic energy of the oil as it exits from between the belt and the pressing surface of the shoe element. Accordingly, a large part of the kinetic energy of the oil is useful for assisting in evacuating the oil.
In accordance with a preferred embodiment of the invention, the oil evacuation arrangement comprises a container having a bottom and a plurality of wall elements upstanding from the bottom. The excess oil is squirted out from between the belt and pressing surface through the inlet opening into the container. An evacuation duct formed in the bottom of the container evacuates the oil. Accordingly, the hot oil is evacuated before it has an opportunity to conduct a significant amount of heat to other parts of the shoe press unit, thereby allowing the temperature in the shoe press unit to be maintained at a lower level. Furthermore, the oil does not accumulate inside the shoe press unit and hence does not tend to be mixed with air, and the energy consumption of the shoe press unit is reduced because only a very small quantity of oil accumulates in the shoe press unit.
Preferably, the upstream edge region of the shoe element includes a convex curved surface and the belt is arranged to first contact the curved surface along a contact line X that extends in the cross-machine direction of the shoe element. The inlet opening is arranged such that a tangent Tx to the curved surface at the contact line X extends into the inlet opening. More specifically, the tangent Tx preferably either coincides with one of, or is between, two imaginary lines Y1 and Y2 that respectively extend from the contact line X to first and second delimiting surfaces that bound downstream and upstream sides of the inlet opening.
In a preferred embodiment, the oil evacuation arrangement includes at least one partition that extends from a surface of the shoe element to the inlet opening for preventing or at least substantially reducing oil flow between the shoe element and the first delimiting surface of the inlet opening. The partition(s) can be integrally formed with one of the wall elements of the container. The container and partition(s) advantageously can be formed of sheet metal, and hence can be inexpensively made.
The delimiting surface of the inlet opening that is nearest the belt preferably is spaced not more than about 10 mm from the belt during operation, and more preferably is spaced less than 5 mm from the belt. This reduces the likelihood of expelled oil leaking between the belt and the inlet opening. In a preferred embodiment, the inlet opening is spaced about 10-150 mm, and more preferably not more than about 100 mm, from the upstream edge region of the shoe element.
The shoe press unit in a preferred embodiment comprises a closed shoe press unit, and the interior of the shoe press unit has an overpressure relative to the pressure outside the shoe press unit of 10-500 mbar. More preferably, the interior overpressure is below 200 mbar, and most preferably is below 50 mbar.
The oil evacuation duct in the bottom of the container can be connected to a vacuum source outside the shoe press unit to facilitate the evacuation of oil. In one embodiment of the invention, the evacuation duct is received within a recess in the beam of the shoe press unit such that the duct can freely move in the recess relative to the beam.